The invention relates to a method for controlling the manufacturing or finishing process of a fiber web at a transition stage of the process in which an error profile is used for determining at least one control signal for the actuators of the manufacturing or finishing process of a fiber web. The invention also relates to a system for controlling the manufacturing or finishing process of a fiber web at a transition stage of the process, which system comprises at least one actuator for affecting the process and controlling the properties of the web (W), a control unit for controlling the process, said control unit comprising control means arranged to form at least one control signal for at least one actuator and that the control means are arranged to form a new control signal by means of a new correction profile.
In the manufacturing or finishing process of a fiber web, for example in the manufacturing or finishing process of a paper or paperboard web, the properties of paper are constantly monitored by means of on-line measurements. The measurements are conducted in the cross-direction of paper in order to produce the profile of the measured property in the cross direction of the paper. Typically the measurements are performed by means of measuring apparatuses, in which a moving fiber web is measured by means of measuring sensors moving back and forth in its cross direction (CD). The properties to be measured may include for example moisture, caliper, basis weight, ash content, color, opacity, brightness, gloss, or smoothness of the web.
The results obtained from the measuring sensors are used not only for monitoring the properties of paper, but also for controlling the manufacturing and finishing devices of paper. The measurement results are transmitted to a control unit, in which they are utilized to determine control signals for profiling apparatuses belonging to the manufacturing or finishing process of paper and affecting said paper property in the cross direction of the paper web. Each of these profiling apparatuses contains one or several actuators affecting a point corresponding to their location in the cross direction of the paper web. The control profile of the profiling device typically comprises the control signals of the actuators relating thereto.
When controlling cross direction profiles, the processing of signals is typically performed by processing information in profile form. For each variable to be measured an error profile is determined, the error profile being the deviation between the profile formed on the basis of the measurement results and the target profile set for the variable, said error profile describing the error in the adjustment. The purpose of the control is to keep the process as accurately as possible in a state complying with the targets determined for the process. By means of the error profile the control unit forms control commands for one or several profiling devices or actuators that affect the process and bring about a change therein complying with the control commands. The prior art control of a manufacturing or finishing process of a fiber web as described above is shown in a very simplified manner in FIG. 1. The process 1 is controlled by a control unit 2, marked with broken lines in the figure. In the process at least one property of a moving fiber web is measured constantly in its cross direction by means of at least one measuring device 3. The measuring devices may be composed of one or several measuring sensors, which are moved back and forth in the cross direction of the web, across the width of the web. As a measuring device it is also possible to use one or several stationary measuring devices positioned in the cross direction of the web in such a manner that their measuring area covers substantially the entire width of the web. The measurement results M produced by the measuring devices are transmitted to a control unit 2, which contains means for processing the measurement results M and forming the control signals. The control unit comprises comparison means 4 to which the measurement results are input. The target values of the process property are also input in the comparison means. The comparison means compare the measured values of the process with the target values of said process property and form an error profile PD on the basis of the comparison, which profile is sent to the control means 5 of the control unit. The control means 5 contain control algorithms forming control signals C on the basis of the error profile PD, which control signals are sent to one or several actuators 6 affecting said property of the web. The actuators are arranged across the width of the web so that they each have a separate area of influence in the cross direction of the web. The control signals C cause the necessary change in the operation of the actuator 6, thus affecting the manufacturing or finishing process of the fiber web as well as the properties of the web that is being manufactured. The control unit updates the error profile PD for example constantly in accordance with a given measurement cycle, time or control interval, producing the control commands C typically on the basis of the last error profile. The error profile PD can be calculated for example at intervals of two measurement scans across the width of the web. The function of the control unit and the means relating thereto are known as such by a person skilled in the art, and therefore they will not be described in more detail in this context.
One problem in the manufacturing or finishing process of a fiber web are regularly occurring disturbances in the operating stages i.e. transition stages deviating from the normal run. The disturbances are typically similar in similar situations and they produce defects in the web that is being manufactured. As a result of the defects the target quality of the web is not reached and the product produced in the process cannot be delivered to a client, but it is treated as a reject. This is not cost-effective.
The transition stages in which the above-mentioned recurrent errors occur include for example a disturbance in the process, a change in a set value relating to the process, starting up of the process or its parts or deceleration before stopping the process. For example after a break, when the process has been started again, the quality of the product does not typically correspond to the target values set for the product, but the target values are reached only after a while from starting the production. The control unit of the process, the automation systems and the actuators control the process during the entire transition stage, but it takes time to reach acceptable product quality. There have been attempts to shorten the time passed for reaching the target quality in various ways, for example by running the process manually. In the manual run the operator can correct the quality of the product by changing the position of the actuators in a way that deviates from the function of the automatic control.
Publication U.S. Pat. No. 4,874,467 discloses a method for controlling the cross-direction profiles of the properties of the paper web. In the publication the position of the actuators controlling the size of the slice of the headbox is adjusted by means of cross direction profiles measured from the paper. In the method the cross direction profile of a certain property of the paper is measured and compared to the target profile. On the basis of the comparison an error profile is formed, which is used further for determining control commands for the actuators.
Publication FI-115325 discloses a method for controlling the manufacturing process of a web, in which a cross direction profile of a certain property of the paper that is being manufactured is determined and compared with a target profile/and an error profile is formed on the basis of the comparison. In process control a group of process models is used, and each one of them is used together with the error profile to determine control operations for the actuators of the process.
Publication F1 116403 (corresponding international publication WO 02/22949) discloses a method for controlling cross direction properties of a web in a calender. In the method at least one cross-direction profile of a web property is measured and compared with the target profile and an error profile is formed. The control process also utilizes a model predicting the effect of the profiling member to a paper property changing in the calendering, said model forming by means of the error profile a control signal to the actuators affecting the measured property.
The drawback of the methods disclosed in the above-mentioned publications is the restriction relating to the feedback information utilized by them. This restriction is the delay caused by the movement time of the measuring sensors in the cross direction of the web. Thus, in transition stages of the above kind the control unit is not capable of reacting sufficiently fast.
One finishing method of a fiber web is calendering, in which the web is passed through one or more nips formed between two surfaces, typically between roll surfaces rotating against each other. The purpose of the calendering is for instance to compress the paper to increase its density, to balance the caliper variations and to improve the surface properties, for example the smoothness and gloss of the surface. Typically, one of the rolls forming a calendering nip is a hard-faced, heated thermo roll and the other roll is a soft-faced roll whose profile can be adjusted or a polymer roll. The roll whose profile can be adjusted may be for example a variable-crown calender roll containing inside itself one or several profiling members, such as loading elements affecting the shell of the roll radially in the direction of the axis of the roll. The loading elements are typically hydraulic pistons which are pressed against the shell of the roll to form the desired profile for the load, i.e. nip load transmitted via the roll to the nip and further to the paper web to be calendered. Thus, it is at the same time possible to compensate the change in the profile caused by the deflection of the roll. The number of loading elements depends on the width of the roll, and they are typically positioned at intervals of 10 to 20 cm in the direction of the axis of the roll. The loading elements can be controlled separately. The controlling takes place by controlling the oil pressure of the loading elements by means of the control system.
The beginning of the calendering and the starting up of the calender can be mentioned as an example of a transition stage in which rapidly developing disturbances occur in the cross direction caliper profiles of the fiber web. FIG. 2 shows a graph illustrating a typical CD caliper profile of a web, measured a few minutes after starting the calendering. As the graph shows, a strong deviation in the caliper profile of the web occurs on both edge areas of the calender when compared to the caliper profile formed by the central part of the calender. The variations primarily result from irregular changes occurring in the flows and distributions of thermal energy inside the calender rolls. The thermal energy flows in the rolls and the temperatures of the rolls are stabilized in an equilibrium corresponding to the running state in the course of time and the function of the control system as well as the quality of the product improve on an acceptable level. This may take for example approximately 15 minutes. It takes some time after this to reach a completely stable run. Also with calenders, attempts have been made to speed up the recovery of the process by switching off the automatic control of the process and controlling the profiling actuator manually. In manual control the linear load profile of the calendering nip is typically influenced by changing the position of hydraulic actuators in the variable-crown roll forming the calender nip so that the caliper profile in the cross-direction of the web would be as uniform as possible.
By controlling the profiling actuator manually at the transition stage, it is possible to reduce the effect of the disturbance to a certain extent. However, the manual method is always very dependent on the skills and experience of the operator. In practice, it has been discovered that by keeping the automatic control switched off during such a production stage until the direction of the error development changes, the time passed after the operating stage for the recovery of the production is shortened approximately 30%. By predicting the error after or before the operating stage it is possible to attain even better results. However, this result is completely insufficient economically.